The present invention relates to the art of actuators of the motorized reducing-gear type, comprising an electric motor coupled to a speed-reduction mechanism.
By actuator there is understood a means for causing rotary movement of an operating element through a travel of less than 360°. The function of an actuator is to ensure precise and repeatable angular positioning under the control of an operating circuit. It differs from a motor, the function of which is to cause an operating element to turn at a given rate of rotation, in the form of n revolutions per minute, without angular limit.
In the prior art there are known numerous motorized reducing gears used in the automobile sector of valves for control of air flow, gas recirculation or air-conditioning. These valves are usually composed of a d.c. motor with brushes, one or two mechanical reduction stages and a potentiometer for indicating the position of the output shaft. Descriptions of such motorized assemblies can be found in U.S. Pat. No. 5,672,818 or in European Patent 1009089.
The disadvantage of these devices is that they use a commutator motor, the useful life of which is limited in applications exposed to demanding temperature and vibration conditions. Similarly, the potentiometers used heretofore have a limited useful life because of the presence of brushes.
From the prior art there is also known Japanese Patent 09322584, which describes a motor and not an actuator. This motor is provided with a speed-reducing gear that drives a relative and not absolute position transducer.
The transducer described in that prior art document delivers two sinusoidal signals having a phase difference of 90°. These are relative and not absolute position signals.
There is also known European Patent 0856939, which describes not a means for operating the motor from a signal delivered by an absolute position transducer, but an end-of travel detector. It is provided with a motor-control circuit activated by a switching circuit, which in turn is operated by the position transducer, which delivers end-of-travel information.
Thus the manufacturers of this type of valve are all working toward the introduction of “brushless” motors, or in other words synchronous motors that are self-commutated by signals from encoders integrated in the motor, and toward the integration of contactless position transducers (of magnetic, inductive or capacitive type) to replace the brush-type devices.
Brushless d.c. motors of diverse polyphase types have been known in the prior art for many years, and a view of such a three-phase motor (100), described in U.S. Pat. No. 5,880,551 of the Applicant, can be seen in FIG. 1. This FIG. 1 shows the positions of three Hall-effect sensors ((1), (2), (3)), placed inside the toothing (18) for the purpose of delivering, as indicated in FIG. 2, three electrical signals ((4), (5), (6)) with a phase difference of 120 electrical degrees as well as the phase difference of these sensor signals relative to the coupling constants ((7), (8), (9)) of the three phases ((19), (20), (21)) of FIG. 1. The presence of these sensors in the stator part of the motor permits an appreciable space savings, wherein an ideal 120° phase is assured by the positioning between 2 teeth of 2 neighboring phases. The goal of the presence of these 3 sensors is to deliver to an operating electronic unit, referred to as “switching logic” in the text of this patent, information indicating the position of rotor (22) with 5 pairs of poles, shown in FIG. 1. As a function of such rotor-position information, the switching logic will send the necessary instructions to an electronic unit known as the “control logic” which, in the case of a three-phase motor, for example, operates the 6 transistors used (4 transistors for a two-phase motor). This operational mode, known as “self-commutation”, makes it possible to achieve optimal adjustment of the current phase difference relative to the coupling constants of each phase (since it takes place automatically relative to the rotor position) and to minimize the torque fluctuation that would be subsequently detrimental to positioning servo control.
Such assemblies require an electronic control circuit that receives on the one hand an input signal originating from the transducer for indicating the angular position of the output shaft and on the other hand the input signals originating from each of the Hall sensors.
FIG. 3 shows a general diagram of the control electronics of a brushless motorized reducing gear integrated in a valve-control application known from the prior art: by means of a PWM (pulse width modulated) signal (15) and an operating-direction signal (16), the control logic (13) will therefore have to process the information originating from the microcontroller (10), responsible for managing positioning servo control by virtue of an analog-to-digital converter (11) that acquires the signal of the output potentiometer (12), and the information originating from the switching logic (14), which delivers information relating to the position of the rotor (22). The control logic (13) will operate the power stage (33) responsible for imposing the current in each phase ((19), (20), (21)) and for driving the rotor (22), engaged with the input pinion of the speed-reducing gear (23), which drives the output shaft, on which there is mounted the absolute position transducer (24). What must therefore be imagined are two interconnected servo control loops:
a) a positioning servo control loop in which the microcontroller will on the one hand modulate, by means of the PWM signal acting on the control logic and therefore on the transistors (33), the level of the current passing through the motor phases, and will on the other hand define, by means of the direction signal acting on the switching logic (14), the direction of rotation of the motor. This positioning servo control loop is of a type identical to that of a servo control loop for d.c. motors.
b) A self-commutation loop, which will act on the control logic (13) by means of the switching logic (14), and will permit supply to the phases in a sequence that is a function of the state of the signals of sensors SA, SB, SC ((4), (5), (6)).
The detail of how these two loops are interconnected is shown in FIG. 4, where the control logic is represented by ET logic gates operating the 6 transistors.
The disadvantage of these prior art devices, self-commutated by 3 sensors, is the large number of conductors necessary. Each of these sensors has 3 connecting conductors (+5 V, ground and signal), the three-phase motor has at least 3 connecting conductors, and the redundant potentiometer may have 4 conductors (+5 V, ground and 2 outputs). Applications with the three-phase brushless d.c. motor of FIG. 1 may therefore be connected by 10 conductors (+5 V, ground, 3 sensor signals, 3 motor connection conductors, 2 redundant potentiometer signals), whereas the same application with brush-type d.c. motor as described in U.S. Pat. No. 5,672,818 of Bosch will have only 6 conductors.
The goal of the present invention is to avoid the disadvantages both of actuators with brush-type motors and of actuators with brushless motors, by proposing an actuator provided with a brushless motor that requires only a reduced number of wiring conductors.